Of all the stars in the Milky Way, red dwarfs – those considerably smaller and dimmer than our own Sun – are thought to make up almost three-quarters.
These have then become rather attractive places to look for life. Not only are they more abundant, but their dimness allows us to more easily see and study planets in orbit around them, including in the so-called habitable zone where liquid water and maybe life could exist.
But as these stars have less heat than our Sun, their habitable zones are much closer in. This, coupled with the increased volatility of these stars than their larger counterparts, has led to questions about how habitable planets around them might really be.
Now, a new study has added some gravitas to those questions. Published in The Astrophysical Journal Letters, scientists at NASA have found that stellar eruptions might erode the atmospheres of red dwarf planets, causing molecules such as hydrogen and oxygen to be lost into space.
“The more X-ray and extreme ultraviolet energy there is, the more electrons are generated and the stronger the ion escape effect becomes,” said co-author Alex Glocer, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement. “This effect is very sensitive to the amount of energy the star emits, which means it must play a strong role in determining what is and is not a habitable planet.”
An artist's concept of how red dwarf planets can lose oxygen ions from their star's radiation. NASA Goddard/Conceptual Image Lab/Michael Lentz/Genna Duberstein
To come to their conclusion, the researchers created a model to estimate the rate of oxygen loss from red dwarf planets. When they took into account “superflares”, massive bursts of radiation from the star, they found that a planet could quite easily lose some of the molecules that are part of the building blocks for life.
The effect was much more prominent for young red dwarfs. These stars can produce superflares every day, whereas older red dwarfs of a similar age to our Sun only do so once every 100 years.
“When we look at young red dwarfs in our galaxy, we see they’re much less luminous than our Sun today,” said lead author Vladimir Airapetian, also from NASA Goddard, in the statement. “By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the Sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms.”
This has implications for worlds like Proxima b, a supposedly Earth-like planet orbiting around our nearest star, the red dwarf Proxima Centauri. Airapetian and his team suggest it is hit by intense radiation from superflares almost every two hours, meaning it would have lost its oxygen in just 10 million years.
More and more, it's looking like our Sun is a rather nice star to orbit.